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Poor Drainage Clay Soils

Clay SoilIf your soil is a heavy clay with poor drainage, most trees will have trouble growing even after a few years of promising growth.

To improve the structure and drainage of this soil:

  • add organic matter to break it up
  • perform a dispersive clay test and if the clay is dispersive, add gypsum
  • redirect excessive soil water with swales
  • install slotted drainage pipes or gravel drains
  • dig deep trenches

An alternative is to grow trees that tolerate poor drainage such as:

  • Banana
  • Chocolate Pudding Fruit
  • Loquat
  • Mulberry
  • Pomegranate
  • Red Currant
  • Nashi Pear with D6 Rootstock

Dispersive Clay & Sodic Soils

If your clay contains a significant amount of sodium ions or compounds, it is likely to be dispersive which is know as a sodic soil. When wet, this clay appears to dissolve in the water, but technically speaking, the clay particles are not 'dissolved' or ionically broken apart, but are suspended in the water. If this soil is present on a steep slope or river bank, rain or river water will 'dissolve' it and the soil will wash away. Vegetation on the river bank will help slow this erosion, and also the associated organic matter in the soil will reduce the dispersive nature of the soil.

In the garden, when dispersive clay dries out, it sets into a hard crust where the spaces between particles very close and evenly distributed. There is insufficient space between clay particles for the movement of water, gases and roots, thus growth of plants and soil organisms is inhibited. Adding calcium in the form of gypsum (calcium sulfate dihydrate, CaSO4·2H2O) will help break up this clay. Soils and clays that are not sodic or dispersive will not benefit in the same way, but gypsum can still be used to add calcium to the soil without altering the soil pH.

Test for Dispersive Clay - Emerson Dispersion Test

Moisten a sample of your clay and roll it into a ball about 15-25cm diameter or the size of a golf ball. Gently place the ball in a glass jar of clean water, ideally rain water or distilled water. Leave the jar undisturbed overnight. If the water has become cloudy and the ball has partially or wholly 'dissolved' (dispersed) into the water, then the clay is dispersive and its structure can be improved by the calcium in gypsum. A gentle swish of the jar may see the remainder of the ball disintegrate. If the water remains clear and the clay does not disperse into the water, the clay is not dispersive and gypsum will not improve its structure.

What does Gypsum do?

Clay particle surrounded by cations

When a dispersed sodic soil or clay drys out, the soil particles settle into a homogeneous layer reflecting the former dispersed nature of the particles.

When gypsum is watered into the soil, it releases its calcium as positively charged ions (cations) which bond to the negatively charged ions (anions) of the clay soil particles. The calcium cation, (Ca2+) is 43 times more powerful at flocculating than the sodium cation (Na+). Flocculation is the bonding of soil particles to form larger clumps, (it is a term used often to describe the clearing of suspended clay from farm dams but the effect is the same in soils). While the new larger clumps are more tightly bonded together, the space between these clumps becomes bigger. This allows water, air and plant roots to move around the soil clumps and the resulting soil is described as having improved 'structure'.

A further benefit of using gypsum is that it does not alter pH of the soil and thus does not alter the availability of soil nutrients to plants and soil organisms.

Soil StructureGypsum application rate

The recommended application rate varies depending on the degree of clay and the quality of the gypsum.

For gardens, with a sandy quality gypsum, the typical application rate is 0.5 to 1 or 2kg per square metre, using a greater amount for more for heavier clay. For gardens, with fine white high quality gypsum, the typical application rate is 0.25 to 1kg per square metre. An an application rate of 1kg per square metre is frequently advised.

For agricultural use, the typical application rate is from 2 to 10 tonnes per hectare,
which is equal to 0.2 to 1 kg per square metre.

Adapted from


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